Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

[Ingo Molnar]

* Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote:

> > That's one reason why i dont think it's necessarily a good idea to 
> > group-schedule threads, we dont really want to do a per thread group 
> > percpu_alloc().
> 
> I still do not have clear how much overhead this will bring into the 
> table, but I think (like Linus was pointing out) the hierarchy should 
> look like:
> 
> Top (VCPU maybe?)
>     User
>         Process
>             Thread
> 
> The "run_queue" concept (and data) that now is bound to a CPU, need to be 
> replicated in:
> 
> ROOT <- VCPUs add themselves here
>     VCPU <- USERs add themselves here
>         USER <- PROCs add themselves here
>             PROC <- THREADs add themselves here
>                 THREAD (ultimate fine grained scheduling unit)
> 
> So ROOT, VCPU, USER and PROC will have their own "run_queue". Picking 
> up a new task would mean:
> 
> VCPU = ROOT->lookup();
> USER = VCPU->lookup();
> PROC = USER->lookup();
> THREAD = PROC->lookup();
> 
> Run-time statistics should propagate back the other way around.

yeah, but this looks quite bad from an overhead POV ... i think we can 
do alot simpler to solve X and kernel threads prioritization.

> > In fact for threads the _reverse_ problem exists, threaded apps tend 
> > to _strive_ for more performance - hence their desperation of using 
> > the threaded programming model to begin with ;) (just think of media 
> > playback apps which are typically multithreaded)
> 
> The same user nicing two different multi-threaded processes would 
> expect a predictable CPU distribution too. [...]

i disagree that the user 'would expect' this. Some users might. Others 
would say: 'my 10-thread rendering engine is more important than a 
1-thread job because it's using 10 threads for a reason'. And the CFS 
feedback so far strengthens this point: the default behavior of treating 
the thread as a single scheduling (and CPU time accounting) unit works 
pretty well on the desktop.

think about it in another, 'kernel policy' way as well: we'd like to 
_encourage_ more parallel user applications. Hurting them by accounting 
all threads together sends the exact opposite message.

> [...] Doing that efficently (the old per-cpu run-queue is pretty nice 
> from many POVs) is the real challenge.

yeah.

	Ingo
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